In signal transmission systems for transmitting or distributing signals within a broad range of frequencies, broadband amplifiers are commonly used to compensate for attenuation of the signals in the transmission medium. For example, in systems wherein the transmission medium is coaxial cable, the transmitted or distributed signals are attenuated by the cable with the amount of attenuation generally dependent upon the length of the cable. The attenuation of coaxial cable is also frequency dependent so that different frequencies are attenuated by different amounts. Signal attenuation as a function of frequency for coaxial cables is typically of the form A=K .sqroot.f, where f is the frequency and K is a constant depending upon the cable type and overall length.
Typical broadband amplifiers for amplifying the signals transmitted via a coaxial cable accordingly provide an overall gain to compensate for the attenuation. The gain of the amplifier is typically divided into two parts called level compensation and slope compensation. For level compensation the signals are amplified by an amount equal to the attenuation without compensating for the frequency dependency of the attenuation. For slope compensation the frequency characteristic of the amplifier is tilted so that the gain is proportional to the square root of frequency to amplify higher frequencies more than lower frequencies.
In prior art broadband amplifiers using transistors, negative feedback from the collector to the base of the transistor provides an amplifier with a gain that tracks the square root of frequency response over a substantial frequency range to a reasonable approximation. Since a given amplifier may be required to compensate for attenuation by any one of a variety of different coaxial cables and cables of varying lengths, a common prior art practice is to provide a manually adjustable feedback for set-up.
While manual set-up provides a certain measure of flexibility, the attenuation of coaxial cable is also dependent upon changing environmental and atmospheric conditions. For example, higher temperatures typically cause increased attenuation. Accordingly, a common prior art practice is to provide automatic gain control to compensate for these varying conditions. U.S. Pat. No. 3,717,813 to D. Lieberman and R. E. Neuber discloses a modular amplifier station with a provision for automatic gain control to control both level and slope. U.S. Pat. No. 3,800,240 to M. Zelenz discloses circuitry for filtering and amplifying pilot signals used for automatic gain control in a broadband amplifier system. In addition, U.S. Pat. No. 3,755,737 to T. S. Eller discloses an automatic gain control system wherein a plurality of pilot signals of off-set frequencies are used for automatic gain control of return amplifiers. In general, level and slope control signals can be developed from either one or two pilot signals included in the transmitted signals. Other forms of automatic gain control are also known in the prior art.
A particularly troublesome and long-standing problem in the prior art has been providing broadband amplifiers of sufficient bandwidth to cover the frequency range desired. In community antenna television (CATV) systems, the frequency response error generally must be limited to .+-.0.1 dB over the frequency range of interest. With typical prior art broadband amplifiers using the above-mentioned negative feedback, several significant limitations are encountered. For example, in a CATV system with sub-VHF signal channels for the reverse or return direction, the range of cable attenuation values for which the prior art amplifiers provide a flat frequency response, using the .+-.0.1 dB limiting criterion, is generally restricted to cable lengths providing up to 5 dB attenuation at 30 mHz thereby necessitating additional amplifiers to maintain signal levels. Also, the frequency range over which the amplifier will track the cable within the .+-.0.1 dB limits is typically 10 mHz to 35 mHz. Attempts to extend the frequency range in the sub-VHF band to encompass lower frequencies have generally resulted in unsatisfactory performance because prior art broadband amplifiers of the type described do not provide a sufficiently flat response at the lower frequencies.